Reflected Lowitz Arcs

Rare 'Reflected' Lowitz Arcs near Falk�ping, Sweden imaged on January 5, '09 by Lennart Ljuhs. The Lowitz arcs are the faint curved arcs extending upwards from each sundog and outside the 22 degree halo. There are traces of more Lowitz arcs to the right of the right-hand sundog. ©Lennart Ljuhs, shown with permission.

Tobias Lowitz first recorded the arcs that bear his name during the St. Petersburg display of 1790 but they remained elusive and their existence highly controversial until first photographed in the 1990s. Even now there remain aspects that are unexplained.

The traditional description of Lowitz arcs is of ones extending from sundogs upwards and downwards to the 22 degree halo. Up to three arcs, middle, upper and lower are possible from rays passing between faces inclined at 60 degrees to each other through Lowitz oriented crystals (usually plates, see the ray diagram at right).

However, when the sun is low, additional arcs form. Those above the horizon extend upwards from sundogs and outside the 22 degree halo. These are the so-called 'reflected' arcs because they are formed when rays undergo internal reflections from the large basal faces of the plates. A better name is needed because there are many Lowitz arcs formed in this manner and they are distinguished by whether the number of internal reflections is odd or even (or zero) and whether they are middle (M), upper (U) or lower (L)ray paths. Lowitz was a professor of chemistry at St. Petersburg and we could perhaps honour him by following chemical nomenclature and name the components g = gerade for even reflections and u = ungerade for odd.

The HaloSim simulation of Lennart Ljuh's display (at right) is preliminary. Rays from Lowitz oriented crystals appear in red. Very thin regular hexagonal plates with limited rotation about the Lowitz axis (AA) were used. Ljuh's strong arcs are upper 'reflected' arcs with ray paths of 3,1,5. Strong enhancement shows a weak middle arc (M). There are also possibly traces on the right hand side of the lower (L) and lower reflected arc (LR). An improved match with the image might be obtained from less regular hexagonal plates.